Placenta

From Wikipedia, the free encyclopedia

The placenta is an ephemeral (temporary) organ present in female placental vertebrates during gestation (pregnancy), but a placenta has evolved independently also in other animals as well, for instance scorpions and velvet worms. The placenta also has an interesting mythology among various indigenous clans of Indochina.

1 Functions
- 1.1 Filtration and transfer
- 1.2 Metabolic and endocrine activity
2 Delivery
3 Non-humans
4 Species variation
- 4.1 Non-placental mammals
5 Pathology
6 Additional images
7 References
8 External links

[edit] Functions

[edit] Filtration and transfer

The placenta receives nutrients, oxygen, antibodies and hormones from the mother's blood and passes out waste. It forms a barrier, the placental barrier, which filters out some substances which could harm the fetus. Many substances are not filtered out, however, including alcohol and some chemicals associated with smoking cigarettes. Several types of viruses, such as Human Cytomegalovirus, may also cross this barrier; this often leads to various degrees of birth defects in the infant.

[edit] Metabolic and endocrine activity

In addition to the transfer of gases and nutrients, the placenta also has metabolic and endocrine activity. It produces, amongst other hormones, progesterone, which is important in maintaining the pregnancy; somatomammotropin (also known as placental lactogen), which acts to increase the amount of glucose and lipids in the maternal blood; oestrogen; relaxin, and human chorionic gonadotrophin HCG. This results in increased transfer of these nutrients to the fetus and is also the main cause of the increased blood sugar levels seen in pregnancy. The site of the former umbilical cord attachment in the center of the front of the abdomen is known as the umbilicus, navel, or belly-button.

[edit] Delivery

When the fetus is delivered, the placenta is delivered afterwards (and for this reason is often called the afterbirth). After delivery of the fetus the umbilical cord is usually clamped and severed prior to the delivery of the placenta or may be left attached to fall off naturally which is referred to as a Lotus Birth. The placenta is delivered mainly by uterine contractions, but the umbilical cord can be gently pulled while pressure is placed on the abdomen, to speed its delivery. Usually the placenta delivers in 2-10 minutes after delivery of the fetus, but it is considered normal as long as the placenta delivers within 30 minutes. If the placenta fails to deliver in 30 minutes, manual extraction may be required, and in some cases a curettage is necessary to ensure that no remnants of the placenta remain. Risks of retained placenta include hemorrhage and infection.

[edit] Non-humans

In most mammalian species, the mother bites through the cord and consumes the placenta, primarily for the benefit of prostaglandin on the uterus after birth. This is known as Placentophagy.

Top: Human placenta shown a few minutes after birth. The side shown faces the baby with the umbilical cord top right. The white fringe surrounding the bottom is the remnants of the amniotic sac. Bottom: A different placenta displays side that connects to the uterine wall..

[edit] Species variation

All mammals other than monotremes and (most) marsupials utilize placentas in reproduction, and are known as placental mammals. Also, some species of snakes utilize placentas.

The shape and exchanging surfaces of placental mammals varies according to species.

Ruminants have cotyledonary placenta that is really many small placentas where the fetus' cotyledons interface with the dams' caruncle forming a placentome.
Carnivores have a zonary placenta.
Perissodactyles have a micro-cotyledonary that grossly ressembles diffuse placentas.
Primates have discoid placentas.

[edit] Non-placental mammals

The only non-placental mammals are the monotremes, which are egg-laying mammals found only in Australia and New Guinea, and marsupials. (Some marsupials have a rudimentary placenta that functions for only a short time; Molly Kalafut's "About Marsupials"[1] points to the bandicoot as the only living example.) According to The Columbia Encyclopedia[2], marsupials, which are now found primarily in Australia and the surrounding region, have evolved placental analogues in those areas where few native placental mammals arose. In other areas, marsupials were largely displaced by the more efficient reproduction (sexual intercourse) of placental mammals.